The present invention relates to a process for the production of chlorine dioxide from an alkali metal chlorate, a mineral acid and methanol as a reducing agent. The process is carried out in a vessel operated under subatmospheric pressure, whereby water is evaporated and withdrawn together with chlorine dioxide and the alkali metal salt of the mineral acid is crystallized within the reaction vessel and withdrawn therefrom. According to the invention the efficiency of the process is improved by increasing the content of formic acid in the reaction vessel by addition of formic acid.
Chlorine dioxide used as an aqueous solution is of considerable commercial interest, mainly in pulp bleaching but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes by which chlorine dioxide can be efficiently produced.
The predominant chemical reaction involved in such processes is summarized by the formula EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O [1]
The chlorate ions are provided by alkali metal chlorate, preferably sodium chlorate, the chloride ions by alkali metal chloride, preferably sodium chloride, or by hydrogen chloride, and the hydrogen ions by mineral acids, normally sulfuric acid and/or hydrochloric acid. Processes for producing chlorine dioxide are described in e.g. U.S. Pat. Nos. 3,563,702 and 3,864,456.
In existing processes for production of ClO.sub.2 there is often also a by-product Cl.sub.2 formation, due to the use of chloride ions as reducing agent according to formula [1]. This chlorine by-product has formerly been used as such in the paper mills as a bleaching agent in aqueous solution. Today there is a tendency towards a more extensive chlorine dioxide bleaching for environmental reasons and thus there is a decreasing need for chlorine as a bleaching agent.
It is also known to use other reducing agents, which do not produce chlorine as a by-product. In U.S. Pat. No. 3,933,988 sulfur dioxide is used as a reducing agent and in U.S. Pat. Nos. 4,081,520, 4,145,401, 4,465,658 and 4,473,540 methanol is used as reducing agent. The methanol is very poorly utilized in a process according to e.g. U.S. Pat. No. 4,465,658. The consumption of methanol is 190-200 kg/ton produced chlorine dioxide whereas the theoretical consumption is only 79 kg/ton according to the formula EQU 6NaClO.sub.3 +CH.sub.3 OH+4H.sub.2 SO.sub.4 .fwdarw.6ClO.sub.2 +CO.sub.2 +5H.sub.2 O+2(Na.sub.3 H(SO.sub.4).sub.2 [ 2(
Thus only about 40% of the methanol charged are used efficiently in existing processes. A thorough study of the reaction products from earlier known processes shows that parts of the added methanol leave the reactor without having reacted. This loss can be as high as 30 to 40 percent.
However, the direct reaction between chlorate ions and methanol is very slow and the true reducing agent in this case is chloride ions reacting according to [1]. The produced chlorine then reacts with methanol to regenerate chloride ions according to the formula EQU CH.sub.3 OH+3Cl.sub.2 +H.sub.2 O.fwdarw.6Cl.sup.- +CO.sub.2 +6H.sup.+[ 1]
It is therefore often necessary to continuously add a small amount of chloride ions in order to obtain a steady production.
A more efficient process with methanol as a reducing agent is described in U.S. Pat. No. 4,770,868. According to this patent it appears that the methanol losses are strongly dependent on the mode of addition of the methanol to the reactor. According to the U.S. patent an improved yield is obtained by introducing the reducing agent in the crystalization zone of the reactor.
The U.S. Pat No. 4,770,868 shows a considerably improved process, but losses of methanol are still obtained as a result of by-reactions. The main by-reaction takes place according to the following net formula: EQU 12NaClO.sub.3 +3CH.sub.3 OH+8H.sub.2 SO.sub.4 .fwdarw.12ClO.sub.2 +4Na.sub.3 H(SO.sub.4).sub.2 +3HCOOH+9H.sub.2 O [4]
The formed formic acid and the methanol which has not been consumed are by-products which only constitute losses in the system. According to known methods they are condensed together with formed water vapour and are added to the absorption tower for the chlorine dioxide absorption. The formic acid and the methanol which has not reacted are thus incorporated in the obtained chlorine dioxide water, which will give as a result that they after the chlorine on the waste water from the bleach plant. Another draw back with formic acid in the chlorine dioxide water is a reduced stability of the water. U.S. Pat. No. 4,770,868 suggests addition of small amounts of catalysts to influence the oxidation of methanol to carbon dioxide in a favourable way.